The Minnesota Pollution Control Agency's first air quality alert of the warm weather season begins at noon Monday.
The alert is prompted by increased ground-level ozone in the Twin Cities and just about every county west to the border with the Dakotas.
The MPCA's orange level of the air quality index for the affected counties expires at 9:00 p.m. Monday.
That means the air quality is unhealthy for sensitive groups, which include people with heart or lung disease, older adults, children, and people with diabetes.
Those people are encouraged to cut back on heavy activity outside during the alert period.
"The ozone season is kind of a narrow season in May and June, you really have to have a high solar angle," said MPCA meteorologist Matt Taraldsen. "As we get later into the summer that's usually when our wildfire smoke start to appear."
Smoke from wildfires usually becomes a problem in July and August.
"They're both equally as impactful and equally as important, but they tend to occur at different parts of the year," said Taraldsen.